In a mobile communication system, a user equipment can receive information from a base station in downlink and the user equipment can transmit information in uplink. The user equipment performs a random access procedure to establish a connection with a network.
More specifically, if a power of a user equipment is turned on again from a state of power-off or the user equipment newly enters a cell, the user equipment performs an initial cell search job for matching synchronization with a base station and the like. Having completed the cell search, the user equipment initiates a random access procedure in a manner of selecting a cell from identified base stations. As the random access procedure is initiated, a timer (e.g., T300 timer), which has defined a limit time for completing the random access procedure, can be initiated as well. If the random access procedure is not completed until the timer expires, the user equipment performs a cell reselection. In case of performing the cell reselection, the user equipment takes into account signal quality of each cell. A representative example of the signal quality may include reference signal reception power (RSRP) or signal-to-noise ratio (SNR) of each cell.
In case of performing a cell reselection, a general LTE (long term evolution) system defines that a more weighting is given to the RSRP of a cell currently accessed by a user equipment or a cell attempts to be accessed by the user equipment (hereinafter called ‘serving cell’ for clarity) and a handicap is given to the RSRP of a surrounding cell. This is intended to prevent the user equipment from frequently moving (ping pong) between cells.
Yet, although signal quality of a surrounding cell is better than that of a serving cell, if it does not offset the weighting given to the serving cell and the handicap given to the surrounding cell, the user equipment continuously stays at the serving cell. In case of performing a random access procedure in the serving cell to enter a network, because a cell reselection condition is not satisfied despite the signal quality of the surrounding cell is better than that of the serving cell, the signal quality of the serving cell is more degraded by interference of the surrounding cell and the random access procedure cannot be successfully performed. Consequently, if the random access procedure performed in serving cell is continuously failed, the cell reselection for a superior cell may become harder for the user equipment to perform since the cell reselection condition is limited.